The relative rates of formation of three major metabolites from the R and S isomers of propranolol, a widely used B-adrenergic blocker, varies with the metabolite and the treatment of rats. 7-Hydroxypropranolol was formed by liver microsomes from untreated rats but not by those from rats pretreated with phenobarbital or B-naphthoflavone. The relative rate of formation of 7-hydroxypropranolol (R/S) ranged from 1.8-2.1. By contrast the R/S ratios for the formation of 4-hydroxypropranolol and desisopropylpropranolol by liver microsomes from either untreated rats or rats treated with phenobarbital were 1.0. With microsomes from B-napthoflavone treated rats, however, the R/S values ranged from 1.45-1.82 for 4-hydroxy-propranolol and from 1.6-2.0 for desisopropanolol formation.